This invention relates generally to optical communications systems and, more particularly, to optical communications systems employing wavelength-multiplexed optical beams from diode lasers. Semiconductor diode lasers are attractive for space communications systems because of their compactness and relatively low power consumption. However, for distances of up to 40,000 km or more, an average laser power of at least 300 mW is needed to obtain communication rates in the 1 Gbit/s (gigabit per second) range. Single diode lasers are not yet available at this power level. Even with the use of coherent laser arrays to increase power output, there is still a need for a new approach to increase the efficiency of the communication system.
A new approach to wavelength multiplexing of optical beams is disclosed and claimed in the cross-referenced patent application, which teaches how multiple laser beams may be combined to form a colinear set of output beams that can be demultiplexed using similar techniques. Although the cross-referenced application is not, in any sense, prior art to the present invention, a brief description of the wavelength multiplexing technique described in the application is needed by way of background to this invention.
In the cross-referenced application, multiple laser beams of different wavelengths are combined by means of roof prisms or a telescope, and directed onto a diffraction grating at angles of incidence selected to provide identical angles of diffraction for all of the beams. Thus, the multiple beams emerge from the diffraction grating in a coincident or colinear manner, and the combined beams have a total power approximating the sum of the individual beam powers.
One of the difficulties with such a system is that the angular relationships important to colinearity of the output beams are maintained only if the wavelengths are precisely regulated. The cross-referenced application suggested the use of a temperature control loop for this purpose. The diode voltage is sensed periodically and compared with a reference level to develop an error signal. The error signal is used to control the diode temperature, which is directly related to the diode wavelength. The principal drawback to this approach is that it does not take into account any mechanical misalignment of the components. Consequently, there can be loss of colinearity even if the temperatures and wavelengths of the diode lasers are perfectly maintained. The present invention overcomes this disadvantage and provides additional useful features, as will become apparent from the following summary.